The invention relates to glass sheets intended to be thermally toughened and more precisely to glass sheets intended to be fitted into motor vehicles.
Although the invention is not limited to such applications, it will be more particularly described with reference to the production of thermally toughened thin glass sheets, i.e. those having a thickness of less than 2.5 mm. This is because motor-vehicle manufacturers are at the present time increasingly tending to wish to limit the weight corresponding to the glazing, while the glass area of the motor vehicles is increasing. A reduction in the thickness of the glass sheets is therefore needed in order to meet these new requirements.
With regard to the thermal toughening of these glass sheets, and more particularly in order to produce the side windowpanes of motor vehicles, the requirements of European Regulation No. 43, relating to the homologation of safety glazing and of the materials for glazing intended to be fitted into motor vehicles and their trailers, have to be met. According to this regulation, the constraints on toughening must be such that the glazing, in the event of it breaking, does so into a number of fragments which, over any 5xc3x975 cm square, is neither less than 40 nor greater than 350 (the latter number being increased to 400 in the case of glazing having a thickness of less than or equal to 2.5 mm). Again according to these requirements, no fragment must be greater than 3.5 cm2, except possibly in a strip 2 cm in width around the periphery of the glazing and within a 7.5 cm radius around the point of impact, and there must not be any elongate fragment of greater than 7.5 cm.
Conventional toughening plants, especially the devices for bending and toughening glass sheets, by making them run along a roller conveyor having a profile that is curved in the direction in which the glass sheet run, allow 3.2 mm thick glass sheets to be toughened according to European Regulation No. 43 completely satisfactorily.
The abovementioned techniques are known, especially from French Patents FR-B-2,242,219 and FR-B-2,549,465 and consist in making the glass sheets, heated in a horizontal furnace, run between two layers of rollers xe2x80x94or other rotating elements xe2x80x94arranged with a curvilinear profile and passing through a terminal toughening zone. In order to produce side windowpanes, sunroofs or other glazing articles, especially of cylindrical shape, the layers consist of, for example, right cylindrical rods arranged with a circular profile. The layers may also consist of elements giving the glazing a secondary curvature, such as conical elements or else those of the diabolo type or barrel type. This technique allows a very high production capacity since, on the one hand, the glass sheets do not have to be widely spaced, it being possible for one glass sheet to enter the forming zone without any problem while the treatment of the previous sheet has yet to be completed and, on the other hand, if the length of the rollers so allow, two or three glass sheets side by side may be treated simultaneously.
The running speed of the glass plates or sheets is at least 10 cm/s and is about 15 to 25 cm/s. The speed normally does not exceed 30 cm/s in order to allow sufficient toughening time.
When the thickness of the glass sheets decreases, and in order to meet the same toughening standards, the heat-exchange coefficient must be greatly increased. To do this, it is possible to increase the blowing power of the toughening devices. Such modifications entail, on the one hand, major investment and, on the other hand, higher operating costs. Moreover, the increase in the blowing power may impair the optical quality of the glass sheets and/or their flatness.
The inventors were thus tasked with the mission of producing glass sheets toughened according to European Regulation No. 43, having a thickness of less than 2.5 mm on standard toughening plants of the type described above.
Thus, the object of the invention is to provide a glass sheet intended to be thermally toughened, the intrinsic properties of which lead to results in the case of thicknesses of less than 2.5 mm but are equivalent to those usually obtained in the case of thicknesses of greater than 3 mm, with the same cooling devices.
This object is achieved by a glass sheet intended to be thermally toughened, the matrix of which is of the silica-soda-lime type and has an expansion coefficient xcex1 of greater than 100xc3x9710xe2x88x927Kxe2x88x921, a Young""s modulus E of greater than 60 GPa and a thermal conductivity k of less than 0.9 W/m.K.
Such properties actually give the glass sheet the possibility of being thermally toughened according to European Regulation No. 43 when this sheet has a thickness of less than 2.5 mm.